Tetrafluoroethylene copolymer fine powder resin

ABSTRACT

Good extrusion at high reduction ratios is obtained with a non-melt-fabricable tetrafluoroethylene fine powder resin whose particles have an inner portion and an outer portion, each portion being composed of a copolymer of tetrafluoroethylene and a selected comonomer, in which the comonomer content of the copolymer in the inner portion is greater than the comonomer content in the copolymer in the outer portion. Such particles are obtained by polymerizing tetrafluoroethylene and a desired amount of the selected comonomer in an aqueous dispersion medium, then lowering the amount of comonomer available, and continuing the polymerization.

FIELD OF THE INVENTION

This invention relates to dispersions of tetrafluoroethylene resins andfine powders suitable for paste extrusion obtained therefrom; andparticularly to such dispersions and powders which contain atetrafluoroethylene copolymer.

BACKGROUND OF THE INVENTION

Two types of polytetrafluoroethylene resins are available commercially,viz, granular resin and fine powder resin. Granular resin is made bypolymerizing tetrafluoroethylene in an aqueous medium under conditionswhich cause the polymer to coagulate during the polymerization reactionto form particles which generally exceed 500 microns in diameter. Theresin is then comminuted to smaller particle sizes, e.g., 30 to 100microns, for molding by such techniques as preforming and sintering orfor ram extrusion.

The fine powder resin is made by polymerizing tetrafluoroethylene in anaqueous medium under conditions which maintain the polymer dispersed asfine particles 0.05 to 0.5 micron in size in the medium until thepolymerization reaction is completed. The particles in the aqueousdispersion can then be coagulated and dried, and are useful in this formfor paste extrusion.

Two main differences between the processes for making these resins isthat (a) stirring in the granular polymerization system is more vigorousthan in the fine powder polymerization system, causing coagulationduring the polymerization reaction and (b) sufficient dispersing agentis present in the fine powder polymerization system to maintain thepolymer particles dispersed until the polymerization reaction iscompleted, whereas the amount, if any, of dispersing agent present inthe granular polymerization system is insufficient to give this result.

Commercially available fine powder resins are not normally fabricable bycommerical molding and ram extrusion processes used for granular resin;and granular resin is not fabricable by the paste extrusion techniquesby which fine powder resin is most commonly processed.

The present invention arises in the field of fine powder the pasteextrusion of these fine powder resins, the resin is blended withlubricant to form lubricated agglomerates which are precompacted andcharged to an extruder barrel and extruded at about room temperaturethrough a die with a cross-section much smaller than that of the barrel.The resulting extrudate is then heated to remove lubricant and usuallysintered by heating to coalesce the residual resin into an integralmass. A common commercial fine powder paste extrusion application isextrusion onto a wire to insulate the wire. An adverse property of thesefine powder resins is their tendency to develop shear faults or flawswhen extruded as a coating onto the wire at high reduction ratios.(Reduction ratio is the ratio of the cross-sectional area of theextruder barrel to the cross-sectional area of the extruder die.) It hasbeen found that each distinct fine powder resin has a certain maximumreduction ratio above which the resin tends to develop flaws as it isextruded. At higher ratios, the resin may actually shatter as it issintered after it is extruded onto the wire. The reason for theappearance of flaws in the coating as the reduction ratio is increasedis not entirely understood but is believed to be due to the shearstresses built up at the entrance to the die as the reduction ratio isincreased. Resins capable of extrusion at high reduction ratios aredesirable because the higher the ratio, the larger the barrel that canbe used, permitting extrusion of longer continuous lengths of coatedwire without reloading the barrel. Thus, the search for fine powderresins capable of extrusion onto wire at high reduction ratios andexhibiting few or no flaws after sintering is a continuing one.

The search is complicated by two facts. Firstly, many prior art reportsof fine powder resins capable of being examination for flaws occurringduring extrusion of beading--i.e., solid cylindrical extrudate--insteadof on sintered wire coating. The former is a less sensitive test becauseflaws in the beading are detected by visual inspection, while flaws inwire coated for electrical use are found by electrically testing inorder to detect much smaller flaws (whose detection is of importance inelectrical applications). Thus, reports of prior art fine powder resinsthat can be used as high reduction ratios are frequently misleadingbecause they are based on gross visual inspections. Secondly, finepowder resin coatings, as extruded on the wire, are unsintered and uponsintering the coated wire for enduse applications, additional flawsappear in the coating. But much of the past work, as evidenced by theprior art in this area, has not considered the flaws that appear duringsintering and again has erroneously reported fine powder resins of goodextrusion quality having high reduction ratios. In reality, however,because of the flaws appearing during sintering, useful reduction ratiosof such resins produced by such past work are much lower.

In summary, in the past, the quality of a fine powder resin for pasteextrusion has been measured by paste extruding unsintered beading andvisually examining the beading for flaws. As a result, resins have beenreported acceptable for extrusion onto wire at reduction ratios as highas 10,000:1. However, though the correlation of unsintered beadingextrudate having few flaws and sintered wire coating extrudate havingfew flaws may be valid when resins are paste extruded at low reductionratio (e.g., 1950:1 or less), the correlation breaks down when extrusionis carried out at higher reduction ratios. In other words, resins whichthe art has said are extrudable to produce acceptable unsintered beadingat reduction ratios of over 1950, are, in reality, unacceptable toproduce sintered wire coatings at the reduction ratios said to beacceptable. For example, Cardinal et al., U.S. Pat. No. 3,142,665,discloses fine powder resins that are said to produce acceptableunsintered beadings at reduction ratios of 10,000:1 or more; however, asshown in the Comparisons hereinbelow, resin produced according to theCardinal et al. patent has numerous flaws when extruded and sintered onwire at a reduction ratio of only 1930:1. On the other hand, theExamples hereinbelow, e.g., Example 1, show the resins of this inventionhad few flaws when extruded and sintered on wire at a reduction ratio of1930:1 and 2840:1.

SUMMARY OF THE INVENTION

In accordance with this invention, good extruded wire coatings areobtained at high reduction ratios with a fine powder resin whoseparticles have an inner portion and an outer portion, each portion beingcomposed of a non-melt-fabricable copolymer of tetrafluoroethylene and aselected comonomer, in which the comonomer content of the copolymer inthe inner portion is greater than the comonomer content in the copolymerin the outer portion. Such particles are obtained by polymerizingtetrafluoroethylene and a desired amount of the selected comonomer in anaqueous dispersion medium, then lowering the amount of comonomeravailable, and continuing the polymerization. Most conveniently,lowering the amount of comonomer available is carried out simply byventing unreacted monomers from the reaction autoclave and repressuringthe autoclave with tetrafluoroethylene, which, upon restarting thepolymerization, polymerizes with residual comonomer. This aspect of thepolymerization will sometimes be referred to hereafter as thevent-repressure step.

Specifically, the compositions of this invention are:

1. An aqueous polymer dispersion comprising a dispersion ofnon-felt-fabricable tetrafluoroethylene polymer particles in water at asolids content of between about 10 and about 65 weight percent; saidparticles having an average size of between about 0.1 and 0.5μ; saidparticles having at least two portions, an outer portion and an adjacentinner portion, each portion consisting essentially of a copolymer ofunits of tetrafluoroethylene and at least one comonomer having theformula ##STR1## wherein R₁ independently is F or H;

R₂ independently is F or Cl;

R₃ can be Cl, --R_(F), --OR_(F), --R'_(F) H, --OR'_(F) H, --OR'_(F) Cl,--R'_(F) Cl or ##STR2## wherein R_(F) is linear perfluoroalkyl of 1-5carbon atoms, and R'_(F) is linear perfluoroalkylene (perfluorinatedalkane diradical) of 1-5 carbon atoms in which the designatedsubstituent is an omega substituent; and

when R₂ is F, R₁ and R₃ taken together can be ##STR3## or the formula##STR4## wherein R₅ and R₆ are independently --CF₃ or --CClF₂ ; saidinner copolymer portion containing a higher percentage of said comonomerthan the outer copolymer portion; the total comonomer content present inthe particle, the percentage of comonomer present in the copolymer ofeach portion, and the amount of each portion with such particles beingsufficient to produce on AWG 22 wire a sintered coating having no morethan 5 flaws per 100 meters of coated wire when said particles are pasteextruded at a reduction ratio of 2840:1, the flaws being detected bysubjecting the sintered coated wire to a high voltage spark tester at2KV and 300 Hz.

2. A tetrafluoroethylene fine powder resin obtained by coagulating theparticles from the dispersion described above. Generally, the coagulatedparticles are agglomerates of the smaller primary particles in theaqueous dispersion, for the coagulated particles usually have an averagesize between about 350μ and about 800μ.

The process aspect of the invention is a process for preparing anaqueous dispersion of tetrafluoroethylene polymer particles whichcomprises

1. subjecting tetrafluoroethylene and at least one comonomer of theformula ##STR5## wherein R₁ independently is F or H;

R₂ independently is F or Cl;

R₃ can be Cl, --R_(F), --OR_(F), --R'_(F) H, --OR'_(F) H, --OR'_(F) Cl,or --R'_(F) Cl or ##STR6## wherein R_(F) is linear perfluoroalkyl of 1-5carbon atoms, and R'_(F) is linear perfluoroalkylene (perfluorinatedalkane diradical) of 1-5 carbon atoms in which the designatedsubstituent is an omega substituent; and

when R₂ is F, R₁ and R₃ taken together can be ##STR7## or the formula##STR8## wherein R₅ and R₆ are independently --CF₃ or --CClF₂ ; whereinthe mole ratio of the amount of the comonomer to the tetrafluoroethyleneis between about 0.0005 and about 0.05, to polymerizing conditions oftemperature and pressure in an aqueous medium having dissolved therein afree-radical initiator and a dispersing agent and at an agitation levelof from between about 2 to 12 joules/sec.-l. until the polymer solidscontent is between about 20 and 50% of the weight of the resultingdispersion,

2. subjecting the aqueous dispersion obtained in step (1) totetrafluoroethylene and said comonomer in a mole ratio of comonomer totetrafluoroethylene of between about 0.0001 and 0.005 provided saidratio is less than that in step (1) under polymerizing conditions oftemperature and pressure and at an agitation level of from between about2 to 12 joules/sec.-l. until the solids content is between about 35 and65% of the weight of the resulting dispersion and is at least about 15%greater than the solids content of the dispersion obtained in step (1).

In one aspect of the invention, the polymerization in step (a) occurs inthe presence of very small particles, e.g. about 0.03 to 0.12μ inaverage size, of tetrafluoroethylene homopolymer. Thus in such aninstance, the fine powder particles produced will contain a small coreof tetrafluoroethylene homopolymer.

By the term "non-melt-fabricable" is meant a tetrafluoroethylene polymerwhose melt viscosity is so high that the polymer cannot be easilyextruded by melt fabrication techniques. Generally the lower themolecular weight of the copolymer, the lower the melt viscosity. A meltviscosity above which tetrafluoroethylene polymers arenon-melt-fabricable is 1×10⁹ poises. The melt viscosity is measured asdescribed under Specific Melt Viscosity found below.

DESCRIPTION OF THE INVENTION

The comonomers used herein are represented by the formula ##STR9##wherein R₁, R₂ and R₃ are defined as recited above. Preferably R₁ and R₂are fluorine. Preferably also R₃ is --R_(F) or --OR_(F). Representativecomonomers include hexafluoropropylene, perfluoroheptene-1,perfluoromethyl vinyl ether, perfluoropropyl vinyl ether,perfluoropentyl vinyl ether, the omega (w) hydrogen or chloro analogs ofthe foregoing comonomers, dichlorodifluoroethylene,perfluoro(2-methylene-4-methyl-1,3-dioxolane), and the like. PreferablyR₃ is --OR_(F), viz, monomers of the formula F₂ C═CFOR_(F). Theseperfluoro (alkyl vinyl)ethers when used as the comonomer generallyresult in polymer particles having very high thermal stability andalthough the thermal stability of all the copolymers used herein isgood, the ether-containing copolymers can be sintered with good resultsat higher temperatures and therefore can be processed at greater ovenspeeds then can other copolymers.

Generally, to obtain good extrudability, the comonomer content in theparticles will be between 0.005 and 2% by weight, based on weight of thecopolymer. The preferred amount will be smaller if the molecular weightof the comonomer increases.

The particles of the invention are produced by polymerizingtetrafluoroethylene and comonomer in stages whereby each portion ofcopolymer produced, after the first, is attached to and instantlyassociated with the copolymer portion produced during the precedingstage. This sequential stage-by-stage polymerization produces particlesof the invention when the comonomer content is varied fromstage-to-stage as described herein.

Polymerization of tetrafluoroethylene and comonomer in both stages isgenerally carried out in accordance with known procedures. Thus, monomerpressures of 1 to 1000 atmospheres may be employed, although it isgenerally preferred to employ pressures from 1 to 75 atmospheres andmost preferably 20-30 atmospheres, since otherwise expensive highpressure equipment is required to handle the monomer safely. Thereaction temperature is maintained at a temperature ranging from 0° toabout 100° C., preferably 50°-95° C. Higher temperatures can be employedif the pressure is sufficiently high to maintain the reaction medium,i.e., the water, in the liquid phase. Cooling of the reaction mixture isgenerally required, since the polymerization is exothermic.

A wide variety of free radical initiators may be employed in the presentinvention, particularly water soluble organic and inorganic peroxides.Examples include disuccinic acid peroxide, diglutaric acid peroxide,monopersuccinic acid, and ammonium persulfate, among others. Preferredinitiators are ammonium persulfate and disuccinic acid peroxide. Redoxpolymerization initiators such as sodium bisulfite withferricitrophosphates may also be employed as polymerization initiatorsin the present invention. The quantity of the initiator may be variedover a wide range depending on the polymerization rate and degree ofpolymerization desired; but generally from 0.0005% to 0.5% of initiatorby weight of the water is present.

The ratio of water to monomer in the process of the present invention isnot critical but is merely a matter of choice depending upon the size ofthe vessel and other obvious factors. In general, the water is usuallypresent on a weight basis ina ratio of greater than one part of waterper part of monomer and preferably 1.5 to 25 parts of water per part ofmonomer. The water should be free of oxygen and chlorine and preferablyshould be demineralized.

A dispersing agent is present in the polymerization mixture in order toensure the production of an aqueous dispersion. The dispersing agentused in the polymerization may be any suitable water-soluble ionizabledispersing agent which will permit the production of aqueous dispersionsof colloidal polymeric tetrafluoroethylene. Some of the most desirabledispersing agents are those compounds having a solubility of water of atleast 0.1% at 100° C. and comprising an ionic hydrophilic portion and ahydrophobic portion, said latter being a highly fluorinated radicalcontaining at least 6 aliphatic carbon atoms. Such dispersing agents aredisclosed in U.S. Pat. No. 2,559,752, issued to K. L. Berry. Since highspecific melt viscosity resins are to be made in this invention, thedispersing agents should be non-telogenic, which means that they are notsufficiently active as chain transfer agents to reduce the meltviscosity of the copolymer prepared below the desired level.

Examples of the preferred dispersing agents are those water-solublesalts from the group consisting of alkali metal, ammonium andsubstituted ammonium salts of a polyfluoroalkanoic acid having thegeneral formula B(CF₂)_(n) COOH, wherein B is from the group consistingof hydrogen and fluorine and n is an integer from 6 to 20 inclusive.Preferably B is fluorine. Specific examples include potassiumhexadecafluorononanoate, ammonium eicosafluoroundecanoate, ammoniumhexadecafluorononanoate, potassium eicosafluoroundecanoate, sodiumdodecafluoroheptanoate, ammonium perfluoropelargonate, sodiumperfluorocaproate, ammonium perfluorocaprylate, and the like. Mixturesof two or more dispersing agents are also suitable for use in thisinvention.

The amount of the dispersing agent used is not particularly critical andmay vary, for example, from 0.01 to 10% by weight of the water used. Ifit is desired to increase the average resin particle size dispersingagent can be added in increments during the polymerization in the firststage according to the following program: Add at least 0.001 weightpercent dispersing agent before 2 weight percent of polymer solids areformed; use an average of 0.002 to 0.05 weight percent dispersing agentduring the formation of the first 4 weight percent of polymer solids,and use in excess of 0.05 weight percent dispersing agent during theperiod in which the polymer solids exceeds 10 weight percent. Weightpercents are based on water.

Other components of the polymerization mixture which may be presentinclude an initiator activating agent. As the initiator activatingagent, there may be added, although such is not essential to thepolymerization, a small quantity of powdered iron as described in U.S.Pat. No. 2,750,350, issued to A. E. Kroll, June 12, 1956. The ironpowder, commercially available as reduced iron powder, being essentiallypure iron free from oxidation products, can be added to increase therate of polymerization when employed in combination with peroxides. Whenemployed, the quantity of the iron used is generally less than 10 p.p.m.by weight of the water present.

It is also preferred in making the more concentrated dispersion of thisinvention to employ a coagulation inhibitor which can be one of thesaturated hydrocarbons as described in U.S. Pat. No. 2,612,484 issued toG. S. Bankoff. As pointed out in the Bankoff patent, these hydrocarbonsare efficient stabilizing agents against coagulation of the polymer andpermit agitation of the reactants without danger of coagulating thepolymer. These hydrocarbon anticoagulants also help to sequester andremove any coagulated polymer which separates from the aqueous medium.The saturated hydrocarbon compounds which are suitable for this purposeinclude those which have more than 12 carbon atoms and are liquid underpolymerization conditions. Specific examples include octadecane,eicosane, tetradecane, cetane, mixtures of hydrocarbons commonly knownas white oils and paraffin waxes, liquid at the polymerizationtemperature. These hydrocarbons can be added to the aqueous mediumbefore polymerization in proportions of about 0.1 to 12% by weight basedon the water present.

The agitation speed is dependent upon the autoclave and agitatordimensions. Howevere, it can be generally classified as mild. Forexample, an agitation speed of 30-60 r.p.m. is appropriate for a 36,250cc. horizontal autoclave having a length-to-diameter ratio of about 1.5to 1 and provided with a four bladed cage-type agitator running thelength of the autoclave. In general, the degree of agitation during thepolymerization of either stage may be between about 2 and 12joules/sec.-l., preferably 8-12.

To prepare the resin particles of this invention, tetrafluoroethyleneand comonomer can be introduced (e.g., pressured) into the reactionvessel (e.g., an autoclave). Either may be instructed first or both maybe added simultaneously. In addition, the monomers can be added inseveral increments during the course of the polymerization. Howeveradded, the total amount of monomers present during the first stage ofthe polymerization should be sufficient to result in a copolymercontaining between about 0.005 and 2.0%, preferably 0.02 to 0.20% byweight comonomer content. In terms of partial pressure, the comonomercontent in the reaction vessel during this stage should be between about0.05 and 5.0 percent, preferably 0.5 to 1.5% of the total pressure inthe autoclave (which is equivalent to a mole ratio of the amount ofcomonomer to tetrafluoroethylene of 0.0005 to 0.05). Of course, thespecific amounts of comonomer employed will depend on the proportion ofunits derived from its desired in the copolymer, on the particularcomonomer used, and on the polymerization conditions employed.

In one embodiment, the comonomer is not added until up to about 15% ofthe tetrafluoroethylene has been polymerized. Resins produced by thisprocedure according to this invention are frequently white and opaquewhen extruded onto wire.

Polymerization to make the portion of the copolymer that is adjacent tothe outer portion is carried out until the polymer solids content of theaqueous reaction mixture is between about 20 and 50% by weight based onthe weight of the mixture and until the amount of tetrafluoroethylene iscopolymerized form amounts to about 25-85%, preferably 65-75%, by weightof the total amount desired in polymerized form. Stirring is thenstopped, and most of the monomer mixture is removed. Removal isordinarily accomplished by simply venting the reaction vessel until thepressure in the vessel is between about atmospheric pressure and up toabout one-half the polymerization pressure used in this stage of thepolymerization.

The resulting aqueous dispersion is then treated to anotherpolymerization by subjecting it to tetrafluoroethylene monomer andcomonomer (preferably, but not necessarily, the same comonomer used inthe previous stage of the polymerization) in amounts such that thecomonomer content in the copolymer produced in this stage is less thanthe comonomer content in the copolymer produced in the previous stage.Generally the amount of comonomer is the copolymer produced in thisstage (i.e., in the preparation of the outer layer of the particle)should be less than one-half, preferably less than one-fifth and mostpreferably less than one-tenth, the amount in the previous stagecopolymer. Generally, also, the amount of the comonomer present will bean amount sufficient to obtain good few-flaw extrudates mentionedpreviously. For example, when perfluoro(propyl vinyl ester) is thecomonomer, the amount present in copolymerized form in the copolymer ofthis last stage should be less than 0.005 mol percent. The minimumamount of the comonomer present in the vessel, regardless of whether thecopolymer of this last stage is obtained by the vent-repressure step, isan amount required to produce a mole ratio of comonomer totetrafluoroethylene of at least about 0.0001. For hexafluoroethylene theminimum amount will be slightly greater. Preferably, the minimum amountwil be about 0.0002. Polymerization of this last stage is then carriedout under the conditions recited hereinabove until the amount oftetrafluoroethylene is polymerized form in the previous stage representsfrom about 25 to about 85% of the total polymerized tetrafluoroethylenein the particle. Usually at this point the total solids is at leastabout 15% greater than it was at the end of the previous stage, andpreferably is about 25-35% greater.

Upon completion of this last stage of the polymerization, the resultingaqueous dispersion is ordinarily passed into a wax separator where thedispersion is cooled to allow separation of the coagulation inhibitor.The aqueous dispersion can be employed as such for use in dip coating orimpregnating applications. The number average size of the resinparticles in the aqueous dispersion is between about 0.1 and 0.5μ. Thus,the resin is in a colloidal state. The resin particles in the aqueousdispersion can be coagulated by subjecting the dispersion to high shearagitation or by other known methods. Suitable agitation power is fromabout 16 to about 160 joules/sec.-l. the resin particles agglomerateduring coagulation to form agglomerates having a weight average size ofbetween about 350μ and about 800μ. The coagulated particles are thenseparated and dried by ordinary procedures.

A second method for coagulation, which can be used to impart improvedhandling properties to the resin, utilizes a modified version of thevessel described in Example 2 of U.S. Pat. No. 2,593,583. The fourequidistantly spaced baffles are shortened to extend from the bottom ofthe cone to approximately half the height of the vessel and a secondagitator, similar to the single agitator but without the blades beingpitched, is added approximately half way from the bottom of the agitatorshaft to the top of the vessel. In this vessel, coagulation is carriedout with the dispersion temperature between 20°-30° C., the dispersionsolids level between 15-20%, and with the agitator speed between 330-600rpm, depending upon the agglomerate particle characteristics desired.

The fine powder resins of this invention can be paste extruded at higherreduction ratios than fine powder resins of the prior art to producesintered coatings of the resin on wire having fewer flaws in the coatingper length than could the fine powder resins hitherto available. TheExamples which follow illustrate the invention, and the Comparisonswhich follow compare the products and process of the invention with onesoutside the scope of the invention. In the Examples, the standardspecific gravity (SSG), specific melt viscosity (MV), average dispersionparticle size and comonomer content are determined as follows:

Standard Specific Gravity (SSG)

SSG is a means of indirectly measuring the molecular weight of atetrafluoroethylene polymer. Generally, the lower the SSG, the higherthe molecular weight. It is determined by the ratio of weight in air toweight of an equal volume of water at 23° C. of a specimen prepared in astandard manner. In the standard specimen preparation, a 12 gram sampleof dry resin powder is leveled between aluminum foils, in a cylindricalmold 2.73 cm. in diameter, and pressure is gradually applied duringabout 30 seconds to a final pressure of about 352 kg./cm.², which isheld for two minutes. The resulting preform is baked in an air oven at380° C. for 30 minutes after heating from 290° to 380° C./min., cooledat 294° C. at a rate of 1° C. per minute, removed from the oven, andthen conditioned for 3 hours at 23° C.

Specific Melt Viscosity

To obtain specific melt viscosity, the rate of elongation is measuredfor a small strip of resin in creep under a known tensile stress. 12 G.of fine powder resin is placed in a 7.6 cm. diameter mold between 0.152cm. rubber cauls and paper spacers. The mold is then heated at 100° C.for 1 hour. Pressure is then slowly applied on the mold until a value of140.6 kg./cm.² is obtained. This pressure is held for 5 minutes and thenreleased slowly. After the sample disc is removed from the mold andseparated from the cauls and paper spacers, it is sintered at 380° C.for 30 minutes. The oven is then cooled to 290° C. at a rate of about 1°C. a minute and the sample is removed. A crack-free rectangular sliverwith the following dimensions is cut: 0.152 to 0.165 cm. wide, 0.152 to0.165 cm. thick, and at least 6 cm. long. The dimensions are measuredaccurately and the cross-sectional area is calculated. The sample sliveris attached at each end to quartz rods by wrapping with silver-coatedcopper wire.

The distance between wrappings is 4.0 cm. This quartz rod-sampleassembly is placed in a columnar oven where the 4 cm. test length isbrought to a temperature of 380±2° C. A weight is then attached to thebottom quartz rod to give a total weight suspended from the samplesliver of about 4 g. The elongation measurements vs. time are obtained,and the best average value for the slope of the creep curve in theinterval between 30 and 60 minutes is measured. The specific meltviscosity is then calculated from the relationship. ##EQU1## wheren=specific melt viscosity in shear, poises

w=tensile load on sample, g.

L_(T) =length of sample (at 380° C.) cms. (length increases about 8% at380° C. over that at room temperature).

g=gravitational constant, 980 cm./sec.²

(dL_(T) /dt)=rate of elongation of sample under loadslope of elongationvs. time plot, cm./sec.

A_(T) =cross-sectional area of sample (at 380° C.), cm.² (area increasesabout 37% at 380° C. over that at room temperature).

Average Dispersion Particle Size

The average size of particles in the dispersion is determined by arelationship based on light-scattering theory from the percentage ofincident light transmitted at 546 millimicron wavelength through a unitmeasure of a dilute dispersion. Dispersion as received is filteredthrough cheesecloth. Then 5 ml of the filtrate is diluted to 500 ml in avolumetric flask. Some of the diluted dispersion is used to fill asilica cell with a 1 cm path length. For dispersions with particlessizes between 0.17 to 0.26μ, ##EQU2## where A=absorbance relative towater

SG=specific gravity of the dispersion as received

S=% solids of the dispersion as received

These particle size values are in the theory nearly equal to theweight-average particle size as confirmed by ultracentrifuge analysis,and are further in reasonable agreement with those determined directlyby examination of electron micrographs of the particles at 20,000diameters magnification.

Average Coagulated Fine Powder Particle Size, d₅₀

The average particle size of coagulated fine powder is determined asfollows: A weighed sample of polymer is placed on the top screen of anassembly of screens which is then tapped manually with a leather hammer.The weight of sample retained on each screen is determined and thefraction of the original sample retained on each screen is plotted vs.screen opening on a log probability chart and a smooth curve is drawnbetween the points. The d₅₀ particle size is read at the 50% valueprinted on the chart.

Comonomer Content in the Fine Powder Copolymers

The vinyl ether content of fine powder resins of the present inventioncan be determined by infrared analysis. A representative procedure is asfollows: A 1.75 g. sample of dry fine powder resin is leveled betweenpieces of aluminum foil in a cylindrical mold, 2.86 cm. inside diameter.A pressure of 703 kg./cm.² is applied for one minute at ambienttemperature. The pressed crack-free sample, which is about 0.127 cm.thick, is then analyzed by infrared spectroscopy. The sample is scannedfrom 9.6 to 11.4 microns. When the vinyl ether is perfluoro(propyl vinylether), a straight base line is drawn from the absorbance minimum at 9.9microns to that at 11.25 microns. Then the ratio of (1) the absorbancefrom the base line to the minimum at 10.1 microns to (2) the absorbancefrom the base line to the absorbance at 10.7 microns is obtained, andthe actual weight percent perfluoro(propyl vinyl ether) is determined bythe product of the ratio of absorbances and the factor, 0.14. Thisfactor can be used for absorbance ratios up to at least 1.0. Ananalogous procedure may be used for other vinyl ethers, or an estimatecan be made based on reactivity. The hexafluoropropylene content isfound from the product of 0.3 and the ratio of absorbances in theinfrared of a cold-pressed sample of the resin 0.05 cm. thick, at 10.18microns to the infrared absorbance of the sample at 10.7 microns. Ananalogous procedure is used for other fluoroolefins.

For chlorotrifluoroethylene comonomer the absorbance at 10.48μ and10.70μ is measured and the weight percent chlorotrifluoroethylene isdetermined by the produect of the ratio of absorbances and the factor0.052.

Extrusion Performance

The performance of a resin of this invention is evaluated by end usetesting using a tetrafluoroethylene paste extruder, either a DavisElectric Co. extruder or a Jennings extruder. The resin sample is rolledwith a hydrocarbon lubricant (viz, "Isopar" H which is an isoparafinichydrocarbon boiling between 350° and 375° F.) at the desired lubricantlevel for 30 minutes, and then stored at 25° C. for at least 4 hours.The mixture is then performed in a cylinder around a rod of the samediameter as the wire guide so that the preform will conveniently fitover the wire guide in the extruder barrel. The Davis extruder wasfitted with barrels of 50.8 mm. and 68.5 mm., a die of 1.40 mm. diameterand 16.0 mm. land length, with a cone angle of 30° and 9.5 mm. and 15.9mm. mandrels, respectively. The wire used was 19/34 silver-coated,stranded copper having an overall diameter of 0.79 mm. The reductionratio for the two barrels is 1930:1 and 2840:1, respectively.

The wire guide has an internal diameter of 0.89 mm. and an externaldiameter of 1.24 mm. The guide tip clearance is 2.03 mm. The dietemperature is maintained at 50° C. and the wire speed is 23 meters perminute. Linear ram speeds used during extrusion for the Davis extruderwere 1.7 cm./min. at 1930:1 ratio and 1.0 cm./min. at 2840:1 reductionratio. The coated wire passes through five ovens set at 232°, 260°,454°, 427° and 399° C., respectively. The number of defects occuring inthe finished wire is determined by passing the wire through a series ofhigh voltage spark testers set progressively at voltages of 2, 5, and 8KV. These testers use alternating current with a 3000 Hz sine wave.

The Jennings extruder has a 7.62 cm. barrel and a 22.2 mm. mandrel isused with the samme cone and die setup for extrusion on AWG 22 wire at a3990:1 reduction ratio. Linear ram speed was 0.42 cm/min. at 3990reduction ratio. Wire speed was 15 meters/min. All other values andconditions are the same as for the Davis extruder for extrusion on AWG22 wire at a 3990:1 reduction ratio.

The results for each resin of the Examples are shown in Table II wherethe number of flows in 100 meters of extruded sintered resin on wire isshown for voltages of 2, 5 and/or 8 KV. The column "lube (weightpercent)" in Table II indicates the amount of hydrocarbon lubricantbased on the total weight of lubricant and resin prior to extrusion."Press (kg/cm²)" records the extrusion pressure employed and "Length(meters)" indicates the total length of wire that was examined forflaws.

The following Examples illustrate the invention while the Comparisonscompare the resins of the invention with resins of the art.

EXAMPLE 1

The following polymerizaton reaction was conducted: A horizontallydisposed, water/steam jacketed, cylindrical stainless-steel autoclavelocated in a barricade and having a capacity of 36,240 cc. and alength-to-diameter ratio of about 1.5 to 1, and provided with a 4-bladedcage-type agitator rotated at 46 r.p.m. and running the length of theautoclave, was evacuated and then charged with 855 grams of paraffinwax, 21.8 kg. of dimineralized water, and 2.0 grams of ammoniumperfluorocaprylate (C-8 APFC) dispersing agent. The autoclave was thenheated to 65° C., evacuated, and purged with tetrafluoroethylene, afterwhich 0.77 g. ammonium persulfate initiator (APS) dissolved in water wasadded. Then 18 ml. of perfluoropropyl vinyl ether (PPVE) was added, andthe autoclave was pressured over a period of about 2 minutes to 29.2kg./cm.² absolute with tetrafluoroethylene (TFE). Stirring rate andtemperature were maintained until polymerization started as evidenced by0.7 kg./cm.² drop in pressure. The temperature was immediately raised to70° C. where it was maintained throughout the polymerization. AdditionalTFE was then added to maintain the reaction pressure at 29.2 kg./cm.²until a dispersion of approximately 35 percent solids content (totalpolymer plus aqueous medium basis) was obtained. After about 1140 gramsof TFE had been fed following start of the reaction (4.4 percent solidsformed), 860 ml. of 3.1 percent by weight, aqueous solution of C-8 APFCwas injected into the autoclave to stabilize the dispersion.

After 7,720 (W₁) gms. of the TFE had been fed following start of thereaction, the TFE feed to the autoclave was terminated and the agitatorstopped. Approximately five minutes after discontinuing the TFE feed,the gaseous monomers were slowly vented (taking approximately tenminutes) from the autoclave until the autoclave pressure reachedautomospheric pressure. Then the agitator was again turned on, theautoclave pressured with TFE, and the reaction started a second time.Then 2,270 (W₂) grams of additional TFE was fed, the monomer fed to theautoclave was then terminated, and the pressure was allowed to decreaseto 12.3 kg./cm.² before agitation was stopped and the vapor space of thereactor was vented. The polymerization time from the first start-up tothe second time the agitator was turned off was 91 minutes. The reactionrate (space time yield) was 391 g/l-hr. The resulting dispersion wasdischarged and cooled, after which the supernatant solid paraffin waxwas removed, and the dispersion was diluted, brought to a pH of 8 to 9by addition of ammonium hydroxide, and coagulated by the procedure ofExample 2 of U.S. Pat. No. 2,593,583 to Lontz. In this coagulationprocedure, the aqueous dispersion is charged to a coagulation kettle anddiluted to about 15% polymer solids. Agitation was then commenced at 18joules/sec.-l. of dispersion at a temperature of 72° F. After ajelly-like mass of coagulum was obtained, stirring was continued forseven minutes. The coagulated fine powder was separated and dried for 16hours at 150° C. This coagulation and drying procedure is generally usedfor all the resins described in the Examples.

The dispersed resin of this example had an average particle diameter of0.18 micron. The coagulated resin had 16% by weight of its particleslarger than 500μ, 50% larger than 355μ and 82% larger than 250μ. Thecoagulated particles had an SSG of 2.165, and a specific melt viscosityof 0.5×10¹⁰ poises at 380° C. Infrared analysis showed the presence of0.06 weight percent PPVE in the resin. Polymerization conditions andproperties of the resin obtained are summarized in Table I.

Extrusion performance on AWG 22 wire was excellent. With a 5.08 cm.barrel and 1.40 mm. die (1930:1 RR) (181/4 percent Isopar H), the flawcount per 100 meters was zero flaws at a voltage of 2 KV, less than 1flaw at 5 KV, and less than 4 8 KV. With a 6.35 cm. diameter barrel and1.40 mm. die (2840:1 RR), (191/4, 19, 183/4 percent Isopar H), the flawcount per 100 meters was zero flaws at a voltage of 2 KV, less than 1flaw at a voltage of 5 KV and less than 11 flaws at a voltage of 8 KV.

The extrusion conditions and flaw count data are summarized in Table II.

EXAMPLE 2

The polymerization procedure of Example 1 was followed except for thosechanges noted in Table I. The most significant change was the initialaddition of comonomer after 1360 grams of TFE had polymerized. Theproperties of the resin obtained are listed in Table I. As with theresin of Example 1, the extrusion performance on AWG 22 wire using a5.08 cm. barrel (1930:1 RR) was excellent, as seen in Table II by thefew flaws occuring. From this experiment, it may be concluded that thegood results with respect to flaws are not dependent upon the presenceof comonomer during the nucleation step of the polymerization(approximately the first 5% by weight of conversion) nor upon thenecessity of having comonomer present within a small core (<12 weightpercent) of the resin particle.

EXAMPLES 3 and 4

Instead of venting the PPVE-TFE gas phase to atmospheric pressure (1.0kg./cm.² absolute) as was done in Examples 1 and 2, the gases werevented to 1.8 kg./cm.<absolute (Example 3) and 2.8 kg./cm.² absolute(Example 4). Otherwise, the steps of Example 1 were followed, but usingthe amounts, times and pressures shown in Table I. The quantity ofcomonomer remaining in the autoclave after the intermediate vent andrepressure is of course increased over that in Example 1 by a factor of1.8 (Example 3) and 2.8 (Example 4). Comparing Examples 1, 3 and 4, itis seen from Table II that as the amount of comonomer units present inthe outer stage increases, more flaws are observed in the wire coatedproduct, but the product of Example 4 still produces a product with fewflaws.

EXAMPLES 5-7

In these Examples, the general procedure of Example 1 was employed,using the conditions and amounts shown in Table I. The results, as seenin Table II, show that few flaws are obtained while varying the volumeof the inner portion relative to the volume of the particle. The size ofthe inner portion, denoted in Table I by W₁ /W₁ +W₂, ranges from about82% of the particle in Example 5 to about 9% in Example 7.

EXAMPLE 8

In this Example, the general procedure of Example 1 was employed, usingthe conditions and amount shown in Table 1. More PPVE per part of TFEwas used than in any previous Examples, as seen by the greater amount ofcomonomer present in the particles prepared (0.15%). This increasedamount had no adverse effect in view of the few number of flaws observedon sintered coated wire as seen from Table II.

EXAMPLES 9-12

Following the procedure of Example 1 and using the conditions, times,and amounts shown in Table I, resins having few flaws when extruded andsintered onto wire were obtained as seen in Table II when:hexafluoropropylene, HFP, was used as the comonomer in place of PPVE(Example 9); a different initiator, disuccinic peroxide, DSP, was used(Example 10); chlorotrifluoroethylene, CTFE, was used as the comonomerin place of PPVE (Example 11); and perfluoro methyl vinyl ether, PMVE,was used as the comonomer in place of PPVE (Example 12).

COMPARISON A

The polymerization procedure of Example 1 was generally used except thatno vent-repressure step was carried out. 11,800 grams of TFE was fedfollowing start of the polymerization into the autoclave which contained20.9 kg. of demineralized water, using a pressure of 27 kg/cm². Then themonomer feed to the autoclave was terminated, and the pressure wasallowed to decrease to 12 kg./cm.² before agitation was stopped and thevapor space of the reactor was vented. The polymerization time fromstart of the reaction to the time when the agitator was turned off was101 minutes. The reaction rate (space time yield) was 400 g./l-hr. Thedispersed resin had an average particle diameter of 0.18 micron. Thecoagulated resin had an SSG of 2.164, and a specific melt viscosity of0.6×10¹⁰ poises at 380° C. Infrared analysis showed the presence of 0.10weight percent PPVE in the resin. Polymerization conditions andproperties of the resin obtained are summarized in Table I.

The fine powder resin obtained produced a continuous bending extrudate,requiring a steady state extrusion pressure of 270 kg./cm.². However, asseen in Table II, extrusion on AWG 22 wire using a 5.08 cm. barrel(1930:1 RR) was unsuccessful. With lubricant levels of 18 and 181/2percent Isopar H, the flaws at a voltage of 2 KV were continuous.Comparing Example 1 with this Comparison A, it is seen that an improvedPPVE modified fine powder is obtained through venting the TFE and PPVEmonomers, and repressuring and continuing the polymerization with TFEfeed as described in Example 1.

COMPARISON B

The polymerization procedure generally used in Example 2 was followedexcept the TFE and PPVE were not vented until completion of thepolymerization. Conditions, times, amounts and resin properties areshown in Table I. When compared with the resin of Example 2, the resinof this Comparison B has poor extrusion properties, as witnessed by thehigh number of flaws shown in Table II.

COMPARISONS C and D

In these comparisons, no comonomer was used. Thus the polymer resinsprepared were polytetrafluoroethylene homopolymer resins. In ComparisonC, the general procedure of Example 1 was followed, but after venting,more tetrafluoroethylene monomer was pressured into the autoclave whichoriginally did not contain any comonomer.In Comparison D, the generalprocedure of Comparison A was followed and no vent-repressure step wascarried out. The conditions, times and amounts employed are shown inTable I. The poor flaw count results in Table II measured at 8 KV showthat the vent-repressure step has no advantageous effect on the resinsconsisting solely of homopolymer.

COMPARISON E, F and G

These comparisons show that using the comonomers without avent-repressure step, i.e., by using the general procedure of ComparisonA, and using the conditions, times and amounts shown in Table I, resinshaving a high number of flaws when extruded and sintered on wire areobtained (see Table II). These results are to be compared with theresults of the Examples as follows: For HFP comonomer, Example 9 iscompared with Comparison E; for CTEF comonomer, Example 11 is comparedwith Comparison F; and for PMVE comonomer, Example 12 is compared withComparison G.

COMPARISON H

The general procedure of Comparison A was employed, using theconditions, times and amounts set forth in Table I. The conditions forthis experiment were those chosen by selecting a set of conditions fromwithin the scope of U.S. Pat. No. 3,142,665 which produces aparticularly high quality product from among those disclosed in saidU.S. Pat. No. 3,142,665 for extrusion onto wire at high reductionratios. As seen from Table II, more flaws were found in the sinteredcoating of a wire of the resin produced in this Comparison H than werefound in sintered coatings on wire of resins of this invention, e.g.,the resins of Examples 9 or 1.

                                      TABLE I                                     __________________________________________________________________________    Polymerization Conditions and Resin Properties                                __________________________________________________________________________                                        Rxn.                                                                              Rxn.                                  Example                                                                              Initiator                                                                           Comonomer                                                                            Temp                                                                              Pressure                                                                           W.sub.1                                                                           W.sub.2                                                                          Time                                                                              Rate                                  No.      (g)    (ml)                                                                              (°C.)                                                                      (kg/cm.sup.2)                                                                      (g) (g)                                                                              (min)                                                                             (g/l-hr)                              __________________________________________________________________________    1      0.33 APS                                                                            18 PPVE                                                                              70  29   7,720                                                                             2,270                                                                            91  391                                   2      0.55 APS                                                                            18 PPVE.sup.1                                                                        70  29   8,630                                                                             2,720                                                                            108 310                                   3      0.77 APS                                                                            18 PPVE                                                                              70  22-27.sup.2                                                                        7,710                                                                             3,180                                                                            92  320                                   4      0.77 APS                                                                            18 PPVE                                                                              70  22-27.sup.2                                                                        7,710                                                                             3,180                                                                            86  350                                   5      0.77 APS                                                                            18 PPVE                                                                              70  29   8,170                                                                             1,815                                                                            97  400                                   6      0.55 APS                                                                            18 PPVE                                                                              70  29   3,630                                                                             7,720                                                                            73  520                                   7      0.33 APS                                                                            40 PPVE                                                                              70  29     910                                                                             9,080                                                                            108 250                                   8      0.33 APS                                                                            40 PPVE                                                                              70  22-27.sup.2                                                                        7,710                                                                             3,180                                                                            122 250                                   9      0.55 APS                                                                            30 HFP.sup.1                                                                         70  29   8,630                                                                             2,720                                                                            102 370                                   10     52.5 DSP.sup.3 4                                                                    12 PPVE                                                                              100 22-29.sup.2                                                                        7,710                                                                             3,180                                                                            191 160                                   11     1.76 APS                                                                            10.2 CTFE.sup.5                                                                      60  26   7,710                                                                             3,180                                                                            71  420                                   12     1.76 APS                                                                            4.8 PMVE.sup.6                                                                       50  26   7,710                                                                             3,180                                                                            309 100                                   Comparison                                                                    A      0.33 APS                                                                            18 PPVE                                                                              70  27   11,800                                                                            -- 101 400                                   B      0.55 APS                                                                            18 PPVE.sup.1                                                                        70  29   11,350                                                                            -- 85  440                                   C      0.77 APS                                                                            --     70  22-27.sup.2                                                                        7,710                                                                             3,180                                                                            88  340                                   D      0.77 APS                                                                            --     70  22-27.sup.2                                                                        9,260                                                                             -- 46  550                                   E      0.55 APS                                                                            30 HFP.sup.1                                                                         70  29   9,580                                                                             -- 70  400                                   F      1.76 APS                                                                            14.4 CTFE.sup.5                                                                      60  26   10,900                                                                            -- 40  750                                   G      1.76 APS                                                                            7.2 PMVE.sup.6                                                                       60  26   10,900                                                                            -- 236 130                                   H      10.5 DSP.sup.4                                                                      25 HFP 90  29   9,700                                                                             -- 119 260                                   __________________________________________________________________________       No. Examp.                                                                         (μ)SizeDispersionAverage                                                         Sp GrStd                                                                           (poise × 10.sup.-10)Specific MV                                                  (wt %)merComono-                                                                   (kg/cm.sup.2)Pressure.sup.7ExtrusionState                                    teadyAverage                                                                        ##STR10##                               __________________________________________________________________________    1       0.18 2.165   0.5   0.06  770 0.77                                     2       0.22 2.168   1.3   0.05  680 0.76                                     3       0.24 2.172   0.7   0.06  500 0.71                                     4       0.23 2.167   0.9   0.06  460 0.71                                     5       0.20 2.157   0.6   0.08  670 0.82                                     6       0.20 2.166   1.8   0.02  620 0.32                                     7       0.19 2.152   2.5   0.03  620 0.09                                     8       0.17 2.156   2.5   0.15  870 0.71                                     9       0.23 2.178   2.2   2.13  550 0.76                                     10      0.18 2.243   1.0   0.06  750 0.71                                     11      0.26 2.227   4.1   0.04  450 0.71                                     12      0.18 2.217   5.9   0.01-0.03                                                                           800 0.71                                     Comparison                                                                    A       0.18 2.164   0.6   0.10  270 1.0                                      B       0.21 2.167   1.0   0.04  500 1.0                                      C       0.27 2.225   11.5  --    520 0.71                                     D       0.26 2.216   8.4   --    530 1.0                                      E       0.23 2.175   2.3   0.15  640 1.0                                      F       0.23 2.254   1.5   0.06  440 1.0                                      G       0.22 2.225   3.7   0.01-0.03                                                                           430 1.0                                      H       0.17 2.199   2.4   0.11  580 1.0                                      __________________________________________________________________________     .sup.1 Comonomer charged after 1360 g TFE reacted.                            .sup.2 Pressure maintained at 22 kg/cm.sup.2 until 680 g TFE reacted.         .sup.3 After APFC pumped, 500 ml DSP solution (35 g/l) pumped at 25           ml/min.                                                                       .sup.4 0.044 g reduced iron and 0.044 g powdered copper added to charge.      .sup.5 0.6 ml CTFE (-40° C.) charged after each 450 g TFE reacted      prior to vent.                                                                .sup.6 0.3 ml PMVE (-40° C.) charged after each 450 g TFE reacted      prior to vent.                                                                .sup.7 U.S. Pat. No. 3,819,594  Column 5, lines 35-67.                   

                                      TABLE II                                    __________________________________________________________________________    Extrusion Performance                                                         AWG 22 Wire, 1.40 mm Die                                                      Davis Extruder               Davis Extruder                                   5.08 cm Barrel               6.35 cm Barrel                                   1930:1 RR                    2840:1 RR                                        Example                                                                             Lube                                                                              2KV                                                                              5KV                                                                              8KV                                                                              Press.                                                                             Length                                                                             Lube                                                                              2KV                                                                              5KV                                                                              8KV                                                                              Press.                                                                             Length                         No.   (wt %)                                                                            (Flaws/100M)                                                                           (kg/cm.sup.2)                                                                      (meters)                                                                           (wt %)                                                                            (Flaws/100M)                                                                           (kg/cm.sup.2)                                                                      (meters)                       __________________________________________________________________________    1     181/2                                                                             1  3  11      300  191/4                                                                             0  1  4  1110 240                                  181/4                                                                             0  0  3  1000 300  19  0  1  11 1110 250                                  18  1  1  3       260  183/4                                                                             0  1  8  1130 110                            2     181/2                                                                             0  1  8  850  270                                                         181/4                                                                             1  1  7  880  280                                                         18  2  2  10 920  240                                                   3     181/2                                                                             1  1  9  670  200                                                         181/4                                                                             1  1  9  660  230                                                         18  1  2  8  680  210                                                   4     181/2                                                                             2  2  16 630  260  181/2                                                                             2  2  15 810  300                                  181/4                                                                             1  1  20 620  240  18  1  1  7  880  300                                  18  2  3  21 650  240  171/2                                                                             0  0  8  940  160                            5                            191/4                                                                             0  3  C  1000 180                                                         19  2  5  C  1050 240                                                         183/4                                                                             1  3  13 1130 230                            6     181/2                                                                             0  1  15 740  280  191/4                                                                             1  2  11 950  400                                  181/4                                                                             1  1  6  770  310  19           980                                       18  0  1  6  800  250  183/4                                                                             3  5  C  1060 290                            7     181/2                                                                             1  2  9       290  191/4                                                                             5  8  C  1000 220                                  181/4                                                                             1  2  12 820  320  19  2  5  C  1020 230                                  18  3  4  14      250  183/4                                                                             1  4  C  1040  85                            8     181/2                                                                             0  0  3  1030 240                                                         181/4                                                                             0  1  3  1100 320                                                   9     181/2                                                                             0  1  5  670  270                                                         181/4                                                                             1  1  2  690  280                                                         18  1  1  3  740  250                                                   10    181/2                                                                             0  1  7  1000 240                                                         181/4                                                                             0  1  8  1080 300                                                         18  0  0  6  1100 220                                                   11    171/2                                                                             1  4  C  650  250                                                         17  0  1  C  700  140                                                   12    181/2                                                                             2  3  C  1040 240                                                         181/4                                                                             0  0  14 1070 290                                                         18  0  0  14 1110 220                                                   COMPAR-                                                                       ISON                                                                          A     181/2                                                                              C*      560  330                                                         181/4                                                                         18  C        630                                                        B     181/2                                                                             C        560  260                                                         181/4                                                                             C        600  270                                                         18  C        630                                                        C     181/2                                                                             1  2  C  680  280                                                         81/4                                                                              2  4  C  710  310                                                         18  1  2  C  750  220                                                   D     181/2                                                                             0  1  C  740  280                                                         181/4                                                                             1  1  C  780  300                                                         18  1  1  C  810  220                                                   E     181/2                                                                             14 14 C  700  300                                                         181/4                                                                             12 13 C  740  310                                                         18  C  C  C  770  260                                                   F     171/2                                                                             C        630  260                                                         17  C        670  240                                                   G     181/2                                                                         181/4                                                                             C        660  310                                                         18  C        660  240                                                   H     181/2                                                                             2  5  C  700  280  191/4                                                                             C        850  360                                  181/4                                                                             1  1  C  700  300  19  C        920  400                                  18  1  1  C  770  260  183/4                                                                             C        950  290                            __________________________________________________________________________     *Greater than 15 flaws/100m.                                             

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows: .[.1. An aqueous polymerdispersion comprising a dispersion of non-melt-fabricabletetrafluoroethylene polymer particles in water at a solids content ofbetween about 10 and about 65 weight percent; said particles having anaverage size of between about 0.1 and 0.5μ; said particles having atleast two portions, an outer portion and an adjacent inner portion, eachportion consisting essentially of a copolymer of units oftetrafluoroethylene and at least one comonomer having the formula##STR11## wherein R₁ independently if F or H; the non-melt-fabricablenature of the polymer particles..]. .[.2. The aqueous polymer dispersionof claim 1 wherein the comonomer is F₂ C═CFCF₃..]. .[.3. The aqueouspolymer dispersion of claim 1 wherein the comonomer is F₂ C═CFOR_(F)..]..[.4. The aqueous polymer dispersion of claim 1 wherein thee comonomeris F₂ C═CFCl..]. .[.5. The aqueous polymer dispersion of claim 2 whereinthe copolymers of the inner portion and the outer portion combinedcontain between about 0.005 and about 2.0 weight percent comonomer unitsbased on total weight of said particles..]. .[.6. The aqueous polymerdispersion of claim 1 wherein the tetrafluoroethylene in polymerizedform in the inner portion comprises between about 25% and 85% by weightof the total tetrafluoroethylene polymerized in the inner and the outerportions..]. .[.7. The aqueous polymer dispersion of claim 1 whereinsaid particles contain a core of polytetrafluoroethylene of up to 15% byweight based on the weight of the particle..]. .[.8. The aqueous polymerdispersion of claim 7 wherein the comonomer is F₂ C═CFCF₃..]. .[.9. Theaqueous polymer dispersion of claim 8 wherein the comonomer is F₂C═CFOR_(F)..]. .[.10. The aqueous polymer dispersion of claim 8 whereinthe comonomer is F₂ C═CFCl..].
 11. An agglomerated fine powder resinwhose agglomerated particles have an average size of between about 350μand 800μ and are composed of primary particles .[.defined as provided inthe definition of the particles in claim 1.]. .Iadd.having an averagesize of between about 0.1 and 0.5μ; said particles having at least twoportions, an outer portion and an adjacent inner portion, each portionconsisting essentially of a copolymer of units of tetrafluoroethyleneand at least one comonomer having the formula ##STR15## wherein R₁independently is F;R₂ independently is F; R₃ is Cl; said inner copolymerportion containing a higher percentage of said comonomer than the outercopolymer portion; the total comonomer content present in the particle,the amount of comonomer present in the copolymer of each portion, andthe amount of each portion within such particles being sufficient toproduce on AWG 22 wire a sintered coating having no more than 5 flawsper 100 meters of coated wire when said particles are paste extruded ata reduction ratio of 2840:1, the flaws being detected by subjecting thesintered coated wire to a high voltage spark tester at 2 KV and 300 Hz;the copolymers of the inner portion and the outer portion combinedcontaining an amount of comonomer which is low enough to maintain thenon-melt-fabricable nature of the polymer particles.Iaddend.. .[.12. Anagglomerated tetrafluoroethylene fine powder resin whose agglomeratedparticles have an average size of between about 350μ and 800μ and arecomposed of primary particles of claim 2..]. .[.13. An agglomeratedtetrafluoroethylene fine powder resin whose agglomerated particles havean average size of between about 350μ and 800μ and are composed ofprimary particles defined as provided in the difinition of the particlesin claim 3..]. .[.14. An agglomerated tetrafluoroethylene fine powderresin whose agglomerated particles have an average size of between about350μ and 800μ and are composed of primary particles defined as providedin the definition of the particles of claim 4..]. .[.15. An agglomeratedtetrafluoroethylene fine powder resin whose agglomerated particles havean average size of between about 350μ and 800μ and are composed ofprimary particles defined as provided in the definition of the particlesin claim 5..]. .[.16. A process for preparing an aqueous dispersion oftetrafluoroethylene polymer particles which comprises1. subjectingtetrafluoroethylene and at least one comonomer of the formula ##STR16##wherein R₁ independently is F or H; R₂ independently is F or Cl; R₃ canbe Cl, --R_(F), --OR_(F), --R'_(F) H, --OR'_(F) H, --R'_(F) Cl,--OR'_(F) Cl, or ##STR17## wherein R_(F) is linear perfluoroalkyl of 1-5carbon atoms, and R'_(F) is linear perfluoroalkylene (perfluorinatedalkane diradical) of 1-5 carbon atoms in which the designatedsubstituent is an omega substituent; and when R₂ is F, R₁ and R₃ takentogether can be ##STR18## or the formula ##STR19## wherein R₅ and R₆independently are --CF₃ or --CCLF₂ ; wherein the mole ratio of theamount of the comonomer to the tetrafluoroethylene is between about0.0005 and about 0.05, to polymerizing conditions to temperature andpressure in an aqueous medium having dissolved therein a free-radicalinitiator and a dispersing agent and at an agitation level of frombetween about 2 to 12 joules/sec.-l.until the polymer solids content isbetween about 20 and 50% of the weight of the resulting dispersion; 2.subjecting the aqueous dispersion obtained in step (1) totetrafluoroethylene and said comonomer in a mole ratio of comonomer totetrafluoroethylene of between about 0.0001 and 0.005, provided saidratio is less than that in step (1) under polymerizing conditions oftemperature and pressure and at an agitation level of from between about2 to 12 joules/sec.-l. until the solids content is between about 35 and65% of the weight of the resulting dispersion and is at least about 15%greater than the solids content of the dispersion obtained in step(1)..]. .[.17. The process of claim 16 wherein the comonomer is F₂C═CFCF₃..]. .[.18. The process of claim 16 wherein the comonomer is F₂C═CFOR_(F)..]. .[.19. The process of claim 16 wherein the comonomer isF₂ C═CFCl..]. .[.20. The process of claim 16 wherein step (1) is carriedout in the presence of particles of between 0.03 and 0.12μ ofpolytetrafluoroethylene in said aqueous medium..]. .[.21. The process ofclaim 16 wherein in step (2) the mole ratio of comonomer istetrafluoroethylene is between about 0.0002 and 0.005..]. .Iadd.22. Anaqueous dispersion of a tetrafluoroethylene polymer comprising colloidalparticles containing at least 98% by weight of polymerisedtetrafluoroethylene and polymerised therewith at least onecopolymerisable monomer selected from perfluoroalkyl trifluoroethylenesand perfluoroalkoxy trifluoroethylenes each having from 3 to 7 carbonatoms, said particles having inner core and outer shell portions inwhich the weight concentration of copolymerised monomer present in theouter shell portion (relative to the weight concentration oftetrafluoroethylene with which it is polymerised) is less than half thatof the inner core portion..Iaddend. .Iadd.23. An aqueous dispersionaccording to claim 22 in which the polymer forming the shell consists ofat least 99.995% polytetrafluoroethylene..Iaddend. .Iadd.24. An aqueousdispersion according to claim 22 in which the copolymerised monomer ishexafluoropropylene..Iaddend. .Iadd.25. A process for the preparation ofan aqueous dispersion of a tetrafluoroethylene polymer comprisingtetrafluoroethylene and a copolymerisable comonomer selected fromperfluoroalkyl trifluoroethylenes and perfluoroalkoxy trifluoroethyleneshaving from 3 to 7 carbon atoms, which process comprises:a. subjecting amixture of tetrafluoroethylene and comonomer to polymerisation in thepresence of an aqueous phase to produce an aqueous dispersion of acopolymer wherein said copolymer contains at least 98% oftetrafluoroethylene units b. reducing the concentration of comonomer inthe reactants and continuing the polymerisation to produce a finalaqueous dispersion wherein the particles of the final dispersioncomprise an inner core portion produced in (a) and an outer shellportion produced in (b), the concentration of comonomer in said shellportion is less than one half the concentration of the comonomer in saidcore portion..Iaddend. .Iadd.26. A method according to claim 25 in whichthe amount of comonomer is used in step (a) is between 27 parts permillion and 10830 parts per million expressed relative to the weight ofthe aqueous phase..Iaddend. .Iadd.27. A method according to claim 26, inwhich the comonomer is hexafluoropropylene and the amount of comonomerused in step (a) is between 27 parts per million and 10830 parts permillion..Iaddend. .Iadd.28. A method according to claim 25, in which themole ratio of comonomer to tetrafluoroethylene used in step (a) is inthe range 0.0005 to 0.05..Iaddend.